1. Technical Field
The disclosure herein generally relates to earth boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the disclosure relates to rolling cone rock bits and to an improved cutting structure and cutter elements for such bits.
2. Description of the Related Art
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole thus created will have a diameter generally equal to the diameter or “gage” of the drill bit.
An earth-boring bit in common use today includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in their path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones or rolling cone cutters. The borehole is formed as the action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the rolling cone cutters is enhanced by providing the cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits or “insert” bits, while those having teeth formed from the cone material are known as “steel tooth bits.” In each instance, the cutter elements on the rotating cutters break up the formation to form the new borehole by a combination of gouging and scraping or chipping and crushing.
In oil and gas drilling, the cost of drilling a borehole is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer, while maintaining a full diameter borehole.
The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration (“ROP”), as well as its durability. Bit durability is, in part, measured by a bit's ability to “hold gage,” meaning its ability to maintain a full gage borehole over the entire length of the borehole. Gage holding ability is particularly vital in directional drilling applications which have become increasingly important. If gage is not maintained at a relatively constant dimension, it becomes more difficult, and thus more costly, to insert drilling apparatus into the borehole than if the borehole had a uniform diameter. For example, when a new, unworn bit is inserted into an undergage borehole, the new bit will be required to ream the undergage hole as it progresses toward the bottom of the borehole. Thus, by the time it reaches the bottom, the bit may have experienced a substantial amount of wear that it would not have experienced had the prior bit been able to maintain full gage. This unnecessary wear will shorten the bit life of the newly-inserted bit, thus prematurely requiring the time consuming and expensive process of removing the drill string, replacing the worn bit, and another new bit downhole.
The geometry and positioning of the cutter elements upon the cone cutters greatly impact bit durability and ROP, and thus are critical to the success of a particular bit design. To assist in maintaining the gage of a borehole, conventional rolling cone bits typically employ a heel row of hard metal inserts on the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to generally align with and ream the sidewall of the borehole as the bit rotates. The inserts in the heel surface contact the borehole wall with a sliding motion and thus generally may be described as scraping or reaming the borehole sidewall. The heel inserts function to maintain a constant gage and to prevent the erosion and abrasion of the heel surface of the rolling cone. Excessive wear of the heel inserts leads to an underage borehole, decreased ROP, increased loading on the other cutter elements on the bit, and may accelerate wear of the cutter bearing and ultimately lead to bit failure.
In addition to the heel row cutter elements, conventional bits typically include a gage row of cutter elements mounted adjacent to the heel surface but orientated and sized in such a manner so as to cut the corner of the borehole. In this orientation, the gage cutter elements generally are required to cut portions of both the borehole bottom and sidewall. The lower surface of the gage row insert engages the borehole bottom while the radially outermost surface scrapes the sidewall of the borehole. Conventional bits also include a number of additional rows of cutter elements that are located on the cones in rows disposed radially inward from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row or bottomhole cutter elements.
One conventional shape for an insert used to cut the borehole corner is a hemispherical or dome-shaped cutter element. This shape provides substantial strength and durability; however, it lacks aggressiveness as it removes formation material via a rubbing motion and provides little shearing as is useful in increasing the rate of removal of material. While other, sharper and more aggressive shapes potentially could be employed to cut the borehole corner, such shapes are not as durable as the partial dome-shaped cutter element, leading to lower ROP and footage drilled, and possibly requiring a premature trip of the drill string to change the bit. Thus, while they may initially remove material at a faster rate, gage cutter elements having aggressively-shaped cutting surfaces may suffer more damage and breakage compared to rounded, less aggressive cutter elements.
Increasing bit ROP while maintaining good cutter element life to increase the total footage drilled of a bit is an important goal in order to decrease drilling time and recover valuable oil and gas more economically. Accordingly, there remains a need in the art for a drill bit and cutting structure that is durable and will lead to greater ROPs and an increase in footage drilled while maintaining a full gage borehole.